1. Field of the Invention
The present invention relates to an optical information recording and reproducing apparatus for recording information bits in a three-dimensional arrangement, and in particular, to the apparatus capable of recording information bits with good performance.
2. Related Art
A conventional optical information recording/reproducing apparatus which records information bits in a three-dimensional arrangement is disclosed in Japanese Patent Laid-Open Publication No. 6-28672. The conventional optical information recording/reproducing apparatus is shown in FIG. 8. In the information recording method employed in this apparatus, a light beam 102 emitted from a light source 101 is incident to an information recording medium 111 formed of a material of a member (“a recording member”) to be recorded with information bits, such as a photopolymer, having a thickness larger than the wavelength of the light beam by means of an objective lens 114 (a converging light beam 107), and thus a converging point is recorded as an information bit 105. The information bit results from a change in the refractive index or the like, and the size of which is generally larger than the wavelength of the light beam in any of the x, y and z directions.
Furthermore, it is possible to reproduce a signal by converging a light beam with low power on this information bit 105 and detecting the converging light beam 107 at a light detector 110 via the objective lens 114 and a beam splitter 113. In order to increase the information capacity, as shown in the same drawing, information bits 105 are recorded in the information recording medium 111 in a three-dimensional arrangement in the direction of the optical axis (z direction) of the objective lens 114.
The inventors of the present invention found that in the conventional optical information recording/reproducing apparatus which records information bits in a three-dimensional arrangement, however, during recording of information, when the converging light beam 107 from the objective lens passes through information bits 116 that have been already recorded, an aberration occurs in the converging light beam 107 under the influence of the information bits 116, which renders the problem that the light beam becomes difficult to converge to the focus point and the information bit 105 that is to be recorded become blurred. It is considered that the information bits 116 which are thicker than the wavelength of the light beam cause the light to be refracted mainly in the region of the information bits 116 and thus the aberration occurs.